The present invention relates generally to fabrication of metal parts including sheet form and three-dimensional form by diffusion bonding in patterns followed by super plastic inflation. More particularly, it relates to the diffusion bonding of sheets as well as three dimensional structures in patterns which permit expansion by pneumatic means of the unbonded portions of the sheets and three dimension structures, such as turbine blades used for power generation or for aircraft engines.
The formation of articles from sheets by selective bonding of metal areas of the sheets and the selective prevention of bonding in other areas of the sheets is a well-developed art. Numerous articles, such as refrigerator evaporators, have been made in this fashion from lower-melting metals such as aluminum. In such conventional practice, a resist material or stop-off coating is printed on a sheet surface in a pattern and the sheet is bonded to another sheet in those areas which do not have a stop-off on the surface. By this conventional practice, the bonding occurs where no stop-off is present and does not occur where the stop-off is present.
When this conventional practice is applied to higher melting metals, such as titanium base metals superalloys and the like, additional problems arise because of the higher temperature and higher pressures which are needed to form the surface-to-surface metal bonds and also because of the difficulty of defining the patterns where the bond is to be generated and separating these from the areas where bonding is to be avoided.
In addition, where the high melting alloy starting materials are themselves non-planar and where the starting structures themselves have complex shapes such as double curvatures, such as turbine fan blades used in power generation or jet engines, in the bonding areas. The resist or stop-off which is useful in processing lower melting metal, such as aluminum alloys, is not satisfactory for processing the higher melting alloys, such as titanium base alloys.
Moreover, the conventional practice of silk screening the resist out of the starting sheet stock does not work where the starting stock is itself nonplanar or a complex shape in the bonding portions. This conventional practice is not suitable for achieving the high degree of precision required in masking complex curved surfaces, such as the component halves of a multicurved configuration turbine blade.
There continues to be a need for an improved effective way to economically bond together high melting alloy into articles having complex shapes. Such methods desirably would provide the same strength of interface at the bonding surface as achieved by previous methods; would be less expensive to utilize; would be simpler; and would maintain bonded interfaces equivalent to interfaces which are diffusion bonded after acid cleaning.